1. Technical Field
This invention relates to audio systems and, more particularly, to speakerphone circuits which connect to an audio line for providing two-way voice communications.
2. Description of the Prior Art
The use of analog speakerphones have been the primary hands free means of communicating during a telephone conversation for a great number of years. This convenient service has been obtained at the price of some limitations, however.
There are two basic limitations that must be addressed in the design of analog as well as other speakerphones: a tendency for self oscillation or regeneration and the generation of a reverberant return echo to a far-end talker. Both limitations are present because of the high gain needed in both an outgoing or transmit channel and an incoming or receive channel of a speakerphone for acceptable hands free operation. The signal in the transmit channel must be amplified from a microphone associated with the speakerphone to a level high enough to comply with predefined telephone transmit specifications over a telephone's tip-ring connection. And the signal in the receive channel must be amplified from the tip-ring connection to a power level high enough to drive a loudspeaker also associated with the speakerphone. Undesirable coupling between these channels is provided by both a two-wire to four-wire hybrid coupling path and the loudspeaker-to-microphone acoustic coupling path which respectively comprise the electrical and acoustical portions of a local closed loop. This loop will typically have a gain much greater than unity and self oscillation will occur when uncompensated.
Because of the proximity of the loudspeaker to the microphone in most speakerphone arrangements, the speech level at the microphone resulting from speech at the loudspeaker is typically much greater than that produced by the speakerphone user or near-end party. This causes the far-end party's speech emanating from the loudspeaker to be coupled into the microphone and back through the telephone line to the far-end party. The result is a loud and reverberant return echo heard by the far-end party.
Historically, these limitations have been addressed in the design of conventional analog speakerphones. The operation of conventional analog speakerphones is well known and is described in an article by A. Busala, "Fundamental Considerations in the Design of a Voice-Switched Speakerphone," Bell System Technical Journal, Vol. 39, No. 2, March 1960, pp 265-294. Analog speakerphones generally use a switched-loss technique through which the energy of the voice signals in both the transmit and the receive channels are sensed and a switching decision made based upon that information. The voice signal having the highest energy level in either channel will be given a clear talking path and the voice signal in the other channel will be attenuated by having loss switched into its talking path. If voice signals are not present in either the transmit channel or the receive channel, the speakerphone typically goes to an "at rest" mode in which loss is switched into the transmit channel, the receive channel or both channels. The amount of voice switched loss inserted into each talking path by variable loss elements is determined by the margin necessary to guard against local loop self oscillation and is typically set by the position of the speakerphone volume control.
Far-end reverberant return echo is not normally a limitation in the operation of conventional analog speakerphones since more loss is switched into the transmit channel during the reception of receive speech to avoid self oscillation than is necessary to satisfactorily attenuate the return echo. Although these conventional analog speakerphones effectively address the two basic limitations, in so doing they inherently introduce others: noise induced false switching; transmit and/or receive lock-out caused by background conversations or intermittent noise; and initial clipping of syllables. Full duplex or "double talking" is also not possible with these speakerphones since voice switched loss is always inserted into one or the other of the two channels. In recent more sophisticated voice switched speakerphones, which under certain limited ideal electrical and acoustical conditions may operate in a near-full to full duplex mode, this return echo appears as a limitation and has to be addressed in the design of these speakerphones.
Another approach for addressing the basic limitations inherent in speakerphone design is through employing echo cancelers therein. In operation, an echo canceler continuously estimates an impulse response between the speakerphone's loudspeaker and microphone and subtracts an echo estimate from the return path. The theory of operation of echo cancelers and their use in reducing the effects of echoes and acoustic coupling between loudspeakers and microphones in close proximity is described in detail in a number of references. A few of these are: R. Ceruti and F. Pira, "Application of Echo-Canceling Techniques to Audioconference," Proc. of IEEE International Conference on Acoustics, Speech, and Signal Processing, March 1982; O. Horna, "Cancellation of Acoustic Feedback,"COMSAT Technical Review, Vol. 12, Fall 1982. pp. 319-333; Y. Itoh, U. Maruyama, N. Furuya, and T. Araseki, "An Acoustic Echo Canceler for Teleconference," Proc. of IEEE International Communications Conference, June 1985. pp. 46.6.1-46.6.5; and B. Widrow, S. D. Stearns, Adaptive Signal Processing, Prentice-Hall, 1985.
The adaptive filtering techniques of echo cancelers is thus known and has been employed in the transmit and receive channels in an echo canceling speakerphone. Through this approach, local loop loss can be effectively added to both the hybrid and acoustic coupling paths. The time span for the impulse response across the hybrid is typically on the order of 4 milliseconds and this echo may be canceled with a relatively short adaptive filter in an echo canceler which cancels the electrical portion of the loop signal provided through the hybrid coupling path. The impulse response of the acoustic coupling path is typically much longer, however, requiring a very long or cascadable adaptive filter in an echo canceler for canceling the acoustic portion of the loop signal provided through the loudspeaker-to-microphone coupling path. In addition, because acoustic paths are sensitive to any motion, the impulse response can vary in time as the speakerphone user moves, the speakerphone is moved or the acoustic environment changes, all resulting in little or no enhancement. When applied to the acoustic coupling path, therefore, the echo canceling technique can be somewhat unreliable and expensive to implement.